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using number_float_t = NumberFloatType;

The type used to store JSON numbers (floating-point).

RFC 8259 describes numbers as follows:

The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

To store floating-point numbers in C++, a type is defined by the template parameter NumberFloatType which chooses the type to use.


Default type

With the default values for NumberFloatType (double), the default value for number_float_t is double.

Default behavior

  • The restrictions about leading zeros is not enforced in C++. Instead, leading zeros in floating-point literals will be ignored. Internally, the value will be stored as decimal number. For instance, the C++ floating-point literal 01.2 will be serialized to 1.2. During deserialization, leading zeros yield an error.
  • Not-a-number (NaN) values will be serialized to null.


RFC 8259 states:

This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.

This implementation does exactly follow this approach, as it uses double precision floating-point numbers. Note values smaller than -1.79769313486232e+308 and values greater than 1.79769313486232e+308 will be stored as NaN internally and be serialized to null.


Floating-point number values are stored directly inside a basic_json type.



The following code shows that number_float_t is by default, a typedef to double.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
    std::cout << std::boolalpha << std::is_same<double, json::number_float_t>::value << std::endl;



Version history

  • Added in version 1.0.0.

Last update: May 1, 2022